1. Field of the Invention
This invention pertains generally to removing stiction between suspended microstructures and their underlying surfaces, and more particularly to removing stiction in cantilevered structures through laser generated stress waves.
2. Description of Related Art
A well-known problem in the fabrication of MEMS (MicroElectroMechanical Systems) devices from surface micromachining is stiction, which occurs when surface adhesion forces are higher than the mechanical restoring force of the micro-structure. When a device is removed from the aqueous solution after wet etching of an underlying sacrificial layer, the liquid meniscus formed on hydrophilic surfaces pulls the microstructure towards the substrate and stiction occurs. The resulting stiction often leaves the particular microstructure inoperable.
Numerous techniques have been explored to resolve this problem. However, most of these techniques have undesirable trade-offs such as requiring expensive and time-consuming processing, altering the design features, or failing to release all the structures without damage. Techniques such as sublimation and supercritical heating reduce stiction by avoiding the evaporative transition from liquid to vapor that often results in stiction failed structures. Both techniques require time for heating and cooling.
Supercritical heating provides more consistent results but requires a more complicated setup. The use of dimples to reduce the contact surface area, the use of polymer posts to temporarily support the cantilevered structures before being removed by dry etching, and the use of low surface energy self assembled monolayers all add complexity to the design or manufacturing process which limits their use as a universal solution. Using low surface energy rinses such as methanol does not always prevent stiction. Manually removing stiction with probe tips is often risky and is not a batch fabrication process. Clearly there is a need for a quick, simple method for recovering stiction failed devices.
Laser energy has been used to quickly heat up stiction failed structures. Differences in thermal expansion increase the strain energy in beams, which provides the driving force for overcoming the stiction forces. This process requires direct heating of MEMS structures to about 50° C. above the temperature of the substrate. While this relative temperature seems quite low, the absolute temperatures these structures were exposed to were not reported. In any case, directly exposing the MEMS structures to laser energy can cause undesired damage to sensitive structures such as those made out of polymeric and cellular materials in newer types of MEMS devices.